If I read you correctly your known designers are all humans. That does mean that you must be cautious about extending the idea to any designer that is presumed to be radically different from a human being.

"Similarity" is just too vague - it is the case that similar causes often have "similar" effects (it's an old saw but if we keep it vague it has a lot of truth to it) so it seems likely that any hypothesis explaining the diversity of life would be expected to produce some degree of similarity. Really you should be looking for the sorts of similarity expected from designers rather than other possible causes. And I don't think that you can do better than stylistic and aesthetic considerations.